Craig Newmark is the founder of craigslist, the online classifieds service. He runs a blog, cnewmark.com, and is working with a wide range of groups who are using the web to help each other out.

(CNN) -- We continuously make decisions every day, from which restaurants and dry cleaners to patronize to who to vote for. Sometimes we know enough to make those decisions ourselves, but often we rely on friends, or on friends of friends, or even on strangers.

When it comes to friends of friends or strangers, we need to know a little about them -- mostly, how others perceive their reliability and their reputation. There are a lot of ways to look at reputation, but we'll keep it simple for now.

I'm thinking about explicit or implicit measures of reputation, like who you know and what you do.

Internet-based tools don't change these fundamentals, but everything online has the potential to someday connect everyone, all the time, everywhere. The net is proving to be an effective platform for systems that implicitly or explicitly rate the reputations of individuals, groups or companies.

The best example, and biggest player, is Facebook, which is connecting several hundred million people across the world. People build implicit reputation by posting what matters to them over a period of time, by building a network of friends, and by interacting with them.

Facebook is more than a site -- it's a platform that allows Facebook interaction across the internet.

The history that you build in your Facebook profile shows people that you're probably a real person, since that kind of investment would be too expensive for scammers or astroturfers -- people who conjure phony grassroots organizations to push causes -- to build. They can try, but scammers usually write material that sounds fishy.

Also, your friend network says a lot about who you are, and building that would be even harder for bad guys to pull off.

I'd speculate that one could write software that took a look at your online social network, your interactions, and so on, and calculate some measurement of your reputation. (I haven't heard this idea before, which is an opportunity both good and bad.)

This concentrates a lot of power in Facebook, and that's one reason people are considering alternative social networks, like Diaspora and OpenSocial.

Another approach, the anonymous rating of reputation, is the model at Unvarnished, an online resource for managing, and researching professional reputation. There, you're rated by other members who login via Facebook and give you numerical grades in several areas.

The Unvarnished folks have thought this system out at great length, and believe that most people will be civil and fair. It's an important experiment.

It also carries considerable risk. There's always the possibility that people might try to game the system by inflating their own ratings or disparaging their competition.

Other systems, such as Amazon, Yelp, and Consumer Reports, invite large numbers of people to rate products and services. The ratings are expressions of trust, and over time, reviewers also get reputations by being rated themselves.

In many respects, we're relying on these reputations for recommendations. As with the other systems, however, the risk is that people post dishonest ratings to drive others' reputations up or down.

Other online systems attempt to measure the integrity of politicians. The Sunlight Foundation, sometimes in conjunction with data.gov, coordinates databases of campaign contributions, votes on specific issues and contracts awarded. That way, software can correlate cash contributions from specific industries with politicians voting either favorably toward or against that industry.

Ultimately, the deal is that trust and reputation issues are pervading our online lives the same way as they do everyday offline lives.

The difference is that online, we carry with us a lot of information about ourselves. That's useful for everyone, helping us make better decisions -- but we all have to watch out for scammers and astroturfers.

Disclosure: Newmark is on the board of the Sunlight Foundation and Consumer Reports. The opinions expressed in this commentary are solely those of Craig Newmark.